1. Field of the Invention
The present invention relates to a tunable optical filter(hereinafter, called TOF) having a wavelength selective property used in an optical communication of a wavelength division multiplexing(hereinafter, called WDM) and method fabricating the same and, more particularly, to an optical filter whose sidelobe disturbing characteristics of an optical filter by weighting coupling efficiency between waveguides is controlled upon applying a selective region growth method in a wavelength selective variable semiconductor optical filter and method of fabricating the same.
2. Discription of the Prior Art
Generally, the structure for realizing the optical filter are several, however, there are often used a Grating-Assisted Co-directional Coupler(GACC) filter coupled in vertical using a semiconductor material whose an integration of an optical device is good. In the structure of a conventional grating assisted co-directional coupler optical filter, there is generated an optical couple upon inserting lattices between two optical waveguides as shown in FIG. 1, the light incident in one waveguide is used as a signal light upon being taken out in the other waveguide. At this time, the lattice 13 functions as coupling light between two waveguides, when two waveguides are asymmetric, only any special wavelength is coupled to other waveguide. The grating assisted co-directional coupler as above can change the wavelength of light generated to a second waveguide by changing the refractivity of the waveguide using a current injection or electric field effect. When an i-th wavelength (xcexi) is outputted from the wavelength variable optical filter as above to the second waveguide, if a (i+1)-th or (ixe2x88x921)-th wavelength are outputted to the second waveguide, there is generated an error for detecting an i-th wavelength. Accordingly, to generate only the i-th wavelength to the second waveguide, it is ideal that an optical wavelength spectrum coupled to the second waveguide from the first waveguide must be 0 on the (ixe2x88x921)-th wavelength adjacent to the (i+l)-th wavelength. However, the optical filter structure shown in FIG. 1 has a form such as an output spectrum coupled to the second waveguide is shown in a slender line 1 of FIG. 3 relative to an optical wavelength change and a small peak value 1a called a sidelobe from the right and left of the central wavelength. Since a channel adjacent according to the position of this sidelobe is influenced, the remove of the sidelobe is essential so that the optical filter can be practical. This sidelobe is a cause to keep an optical couple efficiency between two waveguides constant because the distance between two waveguides is constant as shown in FIG. 1.
In conventional, a first and second waveguide were fabricated on the same plane through a photo-mask work such as a semiconductor process in the form of plane. However, in the case of fabricating an optical filter in the plane form, the distance control between two waveguides is easy, however there is a shortcoming that the wide band is very enlarged because the shape of the waveguide is not even and it is difficult to insert the lattice, therefore, there was performed an optical filter fabrication using LiNbO3 single crystal without performing the crystal growth. However, in the case of an optical filter using the semiconductor, because the crystal growth can be possible by an epitaxial growth method, there was possible the optical filter fabrication in the form of a vertical couple, in the case of a crystal growth, it is possible to fabricate an optical filter of an excellent characteristic because the distance between two waveguides is accurately controlled and a parameter control giving an effect on the characteristic of a filter such as a thickness, width of the waveguide is accurately achieved.
It is an object of the present invention to provide an optical filter whose sidelobe is controlled upon changing the waveguide distance as well as accurately controlling a characteristic since a first waveguide and a second waveguide are fabricated in the form of vertical couple to restrain a sidelobe, namely, one of an important characteristic of the filter and improve the characteristic of the optical filter due to providing a spatial additional value with an optical couple efficiency by means of a selective area growth(SAG) between two waveguides and method of fabricating the same.
In order to accomplish the above object, the present invention provides a grating assisted vertical coupling optical filter whose sidelobe is controlled. Such an optical filter comprises: an n-type InP substrate; an InGaAsP first waveguide layer accumulated in the thickness of 0.3xcx9c0.8 xcexcm on said n-type InP substrate, limited in spatial to have the width of 1xcx9c3 xcexcm and the length of 3xcx9c10 mm; an n-type InGaAsP grating layer formed in the separation of 0.1xcx9c0.5 xcexcm from the first waveguide layer, limited in spatial to have the thickness of 0.02xcx9c0.2 xcexcm and the period of 10xcx9c30 xcexcm; an n-type InP layer accumulated in the thickness of 1xcx9c3 xcexcm on the n-type InGaAsP lattice layer; a p-type InGaAsP second optical waveguide layer accumulated in the thickness of 0.2xcx9c1 xcexcm on said n-type InP, limited in spatial to have the width of 1xcx9c5 xcexcm of and the length of 3xcx9c10 mm; and a p-type InP clad layer accumulated in the thickness of 1xcx9c3 xcexcm on the second optical waveguide layer wherein, the distance between the two waveguides nearest at the middle portion of the optical filter is 0.5xcx9c1.5 xcexcm, and farthest at both ends of the optical filter is 2xcx9c4 xcexcm.
In order to accomplish the above object, the present invention provides a method of fabricating a grating assisted vertical coupling optical filter. Such a method comprises steps of: growing an InGaAsP of material consisting a first waveguide on a InP substrate by an organic metal vapor deposition method(OMVPE), to be patterned to have the width of 1xcx9c3 xcexcm; forming an InP layer and a grating layer thereon, to be patterned to have the period of 10xcx9c40 xcexcm; patterning a dielectric thin film deposited thereon; forming an InP layer thereon by a selective area growth method, and controlling the thickness in spatial; growing an InGaAsP of material consisting a second waveguide on the InP substrate, forming a second waveguide upon being patterned to have the width of 1xcx9c5 xcexcm in the strip form; and controlling a sidelobe upon growing said InP thereon.